Galvanized and galvannealed steel sheets (GA), which are excellent in continuous spot welding and corrosion resistance after painting, are used in a large amount as steel sheets for automobiles. Galvanized and galvannealed steel sheets initially had problems of “powdering” which is an phenomenon that a stiff galvanizing layer is crushed in powder form and exfoliated during press forming in a case that the galvanizing layer is too much alloyed (that is, in a case that the Γ phase that includes body-centered cubic crystal of Zn and Fe intermetallic compound (Fe3Zn10) with 20-28 mass % of Fe is plenty) to make the galvanizing layer stiff. Further, as for a damage of the galvanizing layer, there were problems of “flaking” which is a phenomenon that a galvanizing layer is flaked and exfoliated during press forming under the high surface pressure, in a case that the galvanizing layer is insufficiently alloyed (that is, in a case that the ζ phase that includes monoclinic crystal of Zn and Fe intermetallic compound (FeZn10) with 5.5-6.2 mass % of Fe is plenty) to induce an adhesion between the galvanizing layer and the die or the punch. However, due to advanced galvanizing layer controlling technology and pressing technology, galvanized and galvannealed steel sheets are being used without significant problems. In order to increase the powdering resistance, Γ phase generation in an interface between a galvanizing layer and steel substrate is usually reduced in amount. Meanwhile, in order to increase the flaking resistance, ζ phase in a galvanizing surface layer is usually reduced in amount.
Patent Document 1 discloses a galvanized and galvannealed steel sheet having 1.0 μm or less of Γ phase in an interface between a galvanizing layer and a steel substrate, the galvanizing layer having a galvanizing surface layer which does not include a η phase, which is a hexagonal Zn phase including not more than 0.003 mass % of Fe, or the above-mentioned ζ phase.
Patent Document 2 discloses a galvanized and galvannealed steel sheet having Γ phase in a thickness of not more than 0.5 μm, and having a galvanizing layer which does not include η phase or ζ phase at the galvanizing surface layer.
Patent Document 3 discloses a galvanized and galvannealed steel sheet having a galvanizing layer on a surface of the steel sheet and having a surface roughness Rmax of not greater than 8 μm.
Patent Document 4 discloses a galvanized and galvannealed steel sheet wherein the surface coverage of the ζ phase and the X-ray diffraction intensity ratio between the ζ phase and other phases are determined to be in specific ranges.
Another approach to improve press formability is a series of techniques by providing a lubricating coat on a surface of the galvanized steel sheet instead of controlling the galvanizing layer as described above.
Patent Document 5 discloses a galvanized steel sheet including coats I and II on a galvanizing surface layer, wherein the coat I has an adhesion-preventing function and has one or more metal oxides/hydroxides selected from Mn, Mo, Co, Ni, Ca, Cr, V, W, Ti, Al and Zn as a major component, and wherein the coat II has a rolling-lubricating function and has one or two kinds of oxygen acids selected from P and B as a major component. The coat II gradually increases in concentration toward an interface with galvanizing layer, and the coat II gradually increases in concentration toward a surface of the sheet.
Patent Document 6 discloses a galvanized and galvannealed steel sheet having a flat portion on a surface of an iron-zinc alloy galvanizing layer, the flat portion being provided with an oxide layer which includes: a Zn-based oxide as a major component; a thickness of not less than 8 nm and not more than 200 nm; and an interface width of not less than 25 nm and not more than 500 nm.
Patent Document 7 discloses a galvanized steel sheet which includes a crystalline phosphated coat formed on a surface.